2.1 Fc Receptors and Their Roles in the Immune System
The interaction of antibody-antigen complexes with cells of the immune system results in a wide array of responses, ranging from effector functions such as antibody-dependent cytotoxicity, mast cell degranulation, and phagocytosis to immunomodulatory signals such as regulating lymphocyte proliferation and antibody secretion. All these interactions are initiated through the binding of the Fc domain of antibodies or immune complexes to specialized cell surface receptors on hematopoietic cells. The diversity of cellular responses triggered by antibodies and immune complexes results from the structural heterogeneity of Fc receptors. Fc receptors share structurally related ligand binding domains which presumably mediate intracellular signaling.
The Fc receptors, members of the immunoglobulin gene superfamily of proteins, are surface glycoproteins that can bind the Fc portion of immunoglobulin molecules. Each member of the family recognizes immunoglobulins of one or more isotypes through a recognition domain on the a chain of the Fc receptor. Fc receptors are defined by their specificity for immunoglobulin subtypes. Fc receptors for IgG are referred to as FcγR, for IgE as FFR, and for IgA as FcγR. Different accessory cells bear Fc receptors for antibodies of different isotype, and the isotype of the antibody determines which accessory cells will be engaged in a given response (reviewed by Ravetch J. V. et al. 1991, Annu. Rev. Immunol. 9: 457-92; Gerber J. S. et al. 2001 Microbes and Infection, 3: 131-139; Billadeau D. D. et al. 2002, The Journal of Clinical Investigation, 2(109): 161-1681; Ravetch J. V. et al. 2000, Science, 290: 84-89; Ravetch J. V. et al., 2001 Annu. Rev. Immunol. 19:275-90; Ravetch J. V. 1994, Cell, 78(4): 553-60). The different Fc receptors, the cells that express them, and their isotype specificity is summarized in Table 1 (adapted from Immunobiology: The Immune System in Health and Disease, 4th ed. 1999, Elsevier Science Ltd/Garland Publishing, New York).
Fcγ Receptors
Each member of this family is an integral membrane glycoprotein, possessing extracellular domains related to a C2-set of immunoglobulin-related domains, a single membrane spanning domain and an intracytoplasmic domain of variable length. There are three known FcγRs, designated FcγRI(CD64), FcγRII(CD32), and FcγRIII(CD16). The three receptors are encoded by distinct genes; however, the extensive homology between the three family members suggest they arose from a common progenitor perhaps by gene duplication.
FcγRII(CD32)
FcγRII proteins are 40 KDa integral membrane glycoproteins which bind only the complexed IgG due to a low affinity for monomeric Ig (106 M−1). This receptor is the most widely expressed FcγR, present on all hematopoietic cells, including monocytes, macrophages, B cells, NK cells, neutrophils, mast cells, and platelets. FcγRII has only two immunoglobulin-like regions in its immunoglobulin binding chain and hence a much lower affinity for IgG than FcγRI. There are three human FcγRII genes (FcγRII-A, FcγRII-B, FcγRII-C), all of which bind IgG in aggregates or immune complexes.
Distinct differences within the cytoplasmic domains of FcγRII-A and FcγRII-B create two functionally heterogenous responses to receptor ligation. The fundamental difference is that the A isoform initiates intracellular signaling leading to cell activation such as phagocytosis and respiratory burst, whereas the B isoform initiates inhibitory signals, e.g., inhibiting B-cell activation.
Signaling Through FcγRs
Both activating and inhibitory signals are transduced through the FcγRs following ligation. These diametrically opposing functions result from structural differences among the different receptor isoforms. Two distinct domains within the cytoplasmic signaling domains of the receptor called immunoreceptor tyrosine based activation motifs (ITAMs) or immunoreceptor tyrosine based inhibitory motifs (IFIMS) account for the different responses. The recruitment of different cytoplasmic enzymes to these structures dictates the outcome of the FcγR-mediated cellular responses. ITAM-containing FcγR complexes include FcγRI, FcγRIIA, FcγRIIIA, whereas ITIM-containing complexes only include FcγRIIB.
Human neutrophils express the FcγRIIA gene. FcγRIIA clustering via immune complexes or specific antibody cross-linking serves to aggregate ITAMs along with receptor-associated kinases which facilitate ITAM phosphorylation. ITAM phosphorylation serves as a docking site for Syk kinase, activation of which results in activation of downstream substrates (e.g., PI3K). Cellular activation leads to release of proinflammatory mediators.
The FcγRIIB gene is expressed on B lymphocytes; its extracellular domain is 96% identical to FcγRIIA and binds IgG complexes in an indistinguishable manner. The presence of an ITIM in the cytoplasmic domain of FcγRIIB defines this inhibitory subclass of FcγR. Recently the molecular basis of this inhibition was established. When coligated along with an activating FcγR, the ITIM in FcγRIIB becomes phosphorylated and attracts the SH2 domain of the inosital polyphosphate 5′-phosphatase (SHIP), which hydrolyzes phosphoinositol messengers released as a consequence of ITAM-containing FcγR-mediated tyrosine kinase activation, consequently preventing the influx of intracellular Ca++. Thus crosslinking of FcγRIIB dampens the activating response to FcγR ligation and inhibits cellular responsiveness. B cell activation, B cell proliferation and antibody secretion is thus aborted.
Current approaches to optimize the Fc region function (e.g., antibody-dependent cell mediated cytotoxicity (ADCC), complement dependent cytotoxicity (CDC) activity) in therapeutic monoclonal antibodies and soluble polypeptides fused to Fc regions have focused on a limited number of single amino acid changes based on structural analysis and/or computer aided designs. Alternative approaches in engineering Fc regions have focused on the glycosylation of the Fc region to optimize Fc region function.
TABLE 1Receptors for the Fc Regions of Immunoglobulin IsotypesFcγRIFcγRII-AFcγRII-B2FcγRII-B1FcγRIIIFcαRIReceptor(CD64)(CD32)(CD32)(CD32)(CD16)FcεRI(CD89)BindingIgG1IgG1IgG1IgG1IgG1IgEIgA1, IgA2108 M−12 × 106 M−12 × 106 M−12 × 106 M−15 × 105 M−11010 M−1107 M−1Cell TypeMacrophagesMacrophagesMacrophagesB cellsNK cellsMast cellsMacrophagesNeutrophilsNeutrophilsNeutrophilsMast cellsEosinophilEosinophilNeutrophilsEosinophilsEosinophilsEosinophilsMacrophagesBasophilsEosinophilsDendritic cellsDendritic cellsNeutrophilsPlateletsMast CellsLangerhan cellsEffect ofUptakeUptakeUptakeNo uptakeInduction ofSecretion ofUptakeLigationStimulationGranule releaseInhibition ofInhibition ofKillinggranulesInduction ofActivation ofStimulationStimulationkillingrespiratory burstInduction ofkilling